Optical disks such as compact-disks ROMs (CD-ROMs) and digital-versatile disks (DVDs) store data as pits on the surface for CD or between sandwiched layers for DVD. As a laser scans over these pits in a track, light is scattered, reading a zero, while the absence of pits allows the laser to be reflected back to a photo-sensor, which reads a one.
These pits have a minimum length and spacing. The minimum length for DVD is two bits, while the maximum length is 10 bits. A maximum pit length is needed so that a clock signal can be encoded with the data and extracted during playback.
The pits are sequenced so that analog signals reading the disk do not vary significantly form an average voltage. Minimizing voltage excursions allows more precise small-signal electronics to be used for the read pickup. If the read signal tended to sweep in voltage over time, or had a D.C. buildup, then the small-signal electronics could operate away from its bias point and thus operate with less precision.
Modulation information is added to the user data before recording to achieve a zero D.C. bias. For CD-ROMs, each 8-bit user data byte is expanded during modulation to 14 bits to define the physical pits on the disk. DVDs expand each byte to 16 bits. Rather than simply add 6 or 8 bits to each byte, the byte is completely remapped to a 16-bit codeword that may not have any similarity to the data byte. The modulation codes are carefully chosen to minimize low-frequency or D.C. energy, and include characteristics beneficial to synchronization and merging.
The modulation process for CD-ROMs is known as EFM, for eight-to-fourteen modulation, or simply 8/14. DVDs use a new 8/16 modulation process sometimes called "EFM-Plus", since DVD modulation expands eight bits to sixteen, two more bits than for CD. The modulation codes themselves do not appear to follow any discernable pattern, so tables are often used to convert bytes to codewords and back to bytes. These tables can be quite lengthy, since a one-to-one mapping is not used.
FIG. 1 illustrates modulation of data written to a DVD disk and demodulation of data read from the DVD disk. The user's data is known as a symbol. The user data stream includes the user's actual data and overhead bytes such as error-correction codes, headers, identifiers, and checksums.
A symbol byte SYM is converted to a 16-bit codeword CW by 8/16 modulation, and the 16-bit codeword is written to DVD disk 18. The actual writing process may be quite complex, such as writing a pattern of pits to a substrate, then forming a mold from the patterned substrate, and finally using the mold to stamp out thousands of individual DVD disks 18.
When DVD disk 18 is read, the analog signals are processed to generate a bit stream that is divided into 16-bit codewords. Each codeword is converted by 8/16 demodulation back to an 8-bit symbol SYM, the same symbol that was input to the modulation process when the DVD disk was written.
FIG. 2 shows signals read from a DVD disk. The physical pits on the DVD disk are shown in contour 10. As a laser is scanned over contour 10, it is reflected to a photo-sensor when the surface is flat, such as it is initially in FIG. 2, but scattered when each of the two pits are rotated under the laser. The analog signal with waveform 12 is generated by the photo-sensor and amplified. When the laser is over the pits of contour 10, signal 12 is low (little light picked up by the photo-sensor). When the laser is over the flay surface of contour 10, a high signal 12 is generated by the large amount of laser light reflected to the photo-sensor.
A non-return-to-zero-inverted NRZI encoding is used for the disk data so that signal transitions occur frequently enough to encode a clock with the signal. This clock can be recovered with a phase-locked loop (PLL). Once the data is extracted from the clock from signal 12, the return-to-zero RZ binary bitstream 14 is produced. Each edge of a pit creates a binary one, while no transitions, such as in the middle of a pit or the flat surface, create zeros. As can be seen, bitstream 14 contains many zeros and relatively few ones. The modulated codewords likewise have longer runs of zeros with single ones when the pits transition to the surface.